In digital communication equipment it is often necessary to equalize or adjust the signal level (also referred to as amplitude level) of the received digital signal due to the degradation of signal strength that can occur when transferring the digital signal between different mediums. For example, when digital communication equipment receives or reproduces a digital signal from a magnetic tape, the amplitude (i.e., waveform) of the signal reproduced can vary. The variations in the amplitude of the signal is due to factors such as the characteristics of the recording apparatus or aging effects, for example, the wear of the magnetic head of the reproduction equipment.
In order to compensate for the degradation in the waveform of the reproduced signal, digital systems utilized adaptive equalizing apparatus to automatically restore the waveform of the received signal to a uniform level. In heretofore known adaptive equalizing apparatus the reproduced or received signal was regarded as equivalent to the signal as recorded, and was utilized in the process of automatically equalizing the signal. (see, Miyagawa et al., The Institute of Electronics and Communication Engineers: Digital Signal Processing, p. 233, 1981, 9th edition).
Furthermore, if the degradation of the low frequency component of the received or reproduced signal was extreme, an equalizing apparatus comprising a quantized feedback circuit was employed to correct the signal waveform. The quantized feedback circuit allows for the correction of the low frequency components without an associated increase in noise. (see, Mita et al.: Application of Waveform Equalizing Technology Into Imaging Apparatus, Journal of the Institute of Television Engineers of Japan, Vol. 45, No. 5, pp. 592-600, 1991).
A digital signal reproduction apparatus must be capable of reproducing (from a recording medium) signals recorded by a plurality of different digital signal recording apparatus. The adaptive equalizing apparatus reduces degradation caused by differences in the performance characteristics of the individual recording apparatus utilized to store the data on a recording medium, as well as degradation due to the aging of the reproduction device such as wear of a magnetic head.
FIG. 1 shows an example of a digital signal reproduction apparatus comprising a prior art adaptive equalizing apparatus which includes a quantized feedback circuit. As shown in FIG. 1, a magnetic recording medium 1, which stores digital signals in formats such as non-return-to-zero ("NRZ") and other various modulation techniques, is scanned by a magnetic reproduction apparatus 2 comprising a magnetic head 3. The magnetic head 3 produces a signal, hereinafter referred to as the reproduced signal, corresponding to the information stored on the recording medium 1 which is then input to an integrator 4. The integrator 4, which has high gain in the low frequency band and low gain in the high frequency band, compensates for the low frequency components of the reproduced signal. However, if the low frequency components are only compensated for by the integrator 4, the noise associated with the low frequency components of the reproduced signal may be increased at the same time. As will be described below, a quantized feedback circuit 30 is utilized to compensate for the low frequency components of the reproduced signal so as to prevent an increase of the noise level of the signal.
The output of the integrator 4 is coupled to a finite impulse response ("FIR") filter 10 of an adaptive equalizing apparatus 9. The FIR filter 10 functions to correct the frequency characteristic of the input signal. The FIR filter 10 is coupled to the quantized feedback circuit 30. The FIR filter 10 also receives tap coefficients generated by an algorithm unit 19.
The quantized feedback circuit 30 functions to compensate for the degradation in the low frequency portion of the reproduced signal. The quantized feedback circuit 30 comprises a high-pass filter ("HPF") 31, an adder 32, a comparator 33 and a low-pass filter ("LPF") 34. The HPF 31 receives the output signal of the FIR filter 10 and functions to remove the low frequency component (i.e., components near direct current) of this signal. Preferably, the HPF 31 and the LPF 34 have the same cutoff frequency, for example, 1/100 of the bit rate of the recorded digital data.
The output of the HPF 31 is coupled to a first input of the adder 32. A second input of the adder 32 is coupled to the output of the LPF 34. The output of the adder 32 is coupled to one input of the comparator 33. The comparator 33 compares the signal representing the summation of the output of the HPF and the output of the LPF to a reference signal, which is input to the comparator via a second input terminal, and produces a quantized signal. More specifically, the comparator 33 outputs either a logic high if the level of the output signal of the adder is greater than the reference level, and a logic low otherwise.
As shown in FIG. 1, the output of the comparator 33 is coupled to the input of the LPF 34. As stated above, the output of the LPF 34 is coupled to one of the inputs of the adder 32. The LPF 34 functions to filter the low frequency components out of the quantized signal produced by the comparator 33.
The algorithm unit 19 comprises two inputs for receiving the output signal from the comparator 33 and the output signal from the adder 32. The algorithm unit 19 functions to calculate tap coefficients which are input to the FIR filter 10 during each clock period. One clock period represents one clock cycle of the reproduced digital data and is denoted as period T.
FIGS. 2(a)-(d) illustrate the frequency/power characteristics of the output signals of the various components of the digital signal reproduction apparatus shown in FIG. 1. A digital signal, for example NRZ, recorded in the magnetic recording medium 1 is reproduced by the magnetic head 3 of the magnetic reproduction apparatus 2. The reproduced signal output from the magnetic head 3 generally exhibits a lower power level (i.e., signal strength) in both the low and high frequency bands as shown in FIG. 2(a). The integrator 4 and the FIR filter 10 function to equalize the low and high frequency components of the reproduced signal. As a result, the power levels of the reproduced signal output by the HPF 31 are increased as shown in FIG. 2(b).
The quantized feedback circuit 30 eliminates the noise associated with the low frequency components of the reproduced signal and replaces these low frequency components with the low frequency components generated by the comparator 33. More specifically, the low frequency components of the reproduced signal are filtered by the HPF 31. However, the output of the comparator 33, which comprises low frequency components, is summed with the output of the HPF 31 by adder 32 prior to determining whether the reproduced signal is a logical high or low. Accordingly, the input signal to the comparator 33 comprises both low and high frequency components, and exhibits a reduction in noise.
FIG. 2(c) illustrates the frequency/power characteristics of the low frequency components of the reproduced signal output by the LPF 34. As stated above, the output of the LPF 34 is coupled to the input of the adder 32 and therefore combined with the output of the HPF 31 to form an equalized reproduced signal having corrected power levels in both the high and low frequency bands as shown in FIG. 2(d). The output of the adder 32 is coupled to the comparator and compared with the reference level signal so as to reproduce the original digital data.
Thus, the quantized feedback circuit 30 provides for the compensation of the low frequency components of the reproduced digital signal without an increase in noise levels.
However, such prior art adaptive equalizing apparatus comprising quantized feedback circuits are susceptible to high level noise which results in the abrupt reduction of the tap coefficients of the FIR filter 10. Specifically, the adaptive equalizing apparatus can interpret high level noise as an input signal having a large amplitude, and therefore incorrectly reduce the tap coefficients of the FIR filter 10. Furthermore, the power level of the reproduced signal can be lowered suddenly due to factors including the contamination of the magnetic head 3 or dust deposits on the magnetic recording medium 1. In such cases, the power level of the signal entering the quantized feedback circuit 30 is lowered, and may result in the malfunction of the quantized feedback circuit 30.
FIG. 3 is a signal waveform diagram showing an example of a malfunction of the adaptive equalizing apparatus shown in FIG. 1. FIG. 3(a) illustrates a signal reproduced by the magnetic head 3 and equalized by the FIR filter 10. More specifically, in period T.sub.a a signal having a specified amplitude is entered, and the adaptive equalizing apparatus 9 is working normally. Thereafter, in period T.sub.b, the amplitude of the input signal is lowered suddenly due to any one of the reasons set forth above. As a result of the decrease in amplitude of the input signal, the amplitude of the DC component of the output Y.sub.-1 of the adder 32 is increased during the period T.sub.b, as shown in FIG. 3(b), by the feedback signal from the LPF 34. Specifically, as the comparator outputs consecutive logic highs, the average value of the output signal of the adder 32 (i.e., DC component) gradually approaches the level associated with a logic high as a result of the frequency response of the LPF 34.
Thus, during the next period T.sub.c, if a signal representing a logic low is entered, the amplitude of the output of the adder 32 may not be below the reference level of the comparator 33 (0 volts in FIG. 3). Therefore the output a.sub.-1 of the quantized feedback circuit 30 can incorrectly produce a logic high signal during period T.sub.c, as shown in FIG. 3(c).
The operation of the quantized feedback circuit 30 will only be corrected (i.e., DC component of the output of the adder 32 returned to desired level) if an input signal exceeding the specified amplitude level, or a signal having an amplitude sufficiently lower than the reference level so that the comparator produces a logic low, is entered into the quantized feedback circuit 30.
However, as a result of the shift in the DC component in the output of the adder 32, the input signals Y.sub.-1 and a.sub.-1 to the algorithm unit 19 are different from the original desired values which results in the malfunction of the algorithm unit 19. Thus, the FIR filter 10, which receives tap coefficients from the algorithm unit 19, does not function correctly.
More specifically, as the comparator 33 outputs consecutive logic highs, such as signal a.sub.-1, as shown in FIG. 3(c), the average value of the output of the adder 32, Y.sub.-1, approaches a logic high. As a result, the algorithm unit 19 considers the tap coefficients of the FIR filter 10 to be correct and therefore does not increase the amplitude of the output signal of the FIR filter 10. Thus, adaptive equalizing apparatus 9 does not return to normal operation.
Accordingly, there exists a present need for an adaptive equalizing apparatus comprising a quantized feedback circuit which does not enter a state of permanent malfunction if the amplitude of the input signal to the quantized feedback circuit is lowered suddenly.